Improvement of biological nitrogen fixation in Egyptian winter legumes through better management of Rhizobium

The diversity of rhizobia nodulating common bean (Phaseolus vulgaris), berseem clover (Trifolium alexanderinum) and lentil (Lens culinaris) was assessed using several characterization techniques, including nitrogen fixation efficiency, intrinsic antibiotic-resistance patterns (IAR), plasmid profiles, serological markers and rep-PCR fingerprinting. Wide diversity among indigenous rhizobial populations of the isolates from lentil, bean and clover was found. Strikingly, a large percentage of the indigenous rhizobial population was extremely poor at fixing nitrogen. This emphasizes the need to increase the balance of highly efficient strains within the rhizobial population. Use of high-quality inocula strains that survive and compete with other less-desired and less-efficient N₂-fixing rhizobia represents the best approach to increase biological nitrogen fixation of the target legume. In field-grown lentils, the inoculant strains were not able to outcompete the indigenous rhizobia and the native lentil rhizobia occupied 76-88% of the total nodules formed on inoculated plants. Nitrogen fixation by lentils, estimated using the ¹⁵N isotope dilution technique, ranged between 127 to 139 kg ha⁻¹ in both inoculated and un-inoculated plants. With berseem clover, the inoculant strains were highly competitive against indigenous rhizobia and occupied 52-79% of all nodules. Inoculation with selected inocula improved N₂ fixation by clover from 162 to 205 kg ha⁻¹ in the three cuts as compared with 118 kg ha⁻¹ in the un-inoculated treatment. The results also indicated the potential for improvement of N2 fixation by beans through the application of efficient N₂-fixing rhizobia.